When pressure and flow are measured at the airway opening (i.e. the mouth) in a human subject the values obtained are dependent on: 1. Force-velocity properties of muscle; 2. Passive mechanical properties of the physiological system; 3. Any externally added impedances. The basic experimental procedure in this study is to measure pressure and simultaneous flow at the mouth, immediately following the opening of a shutter against which the subject is making an inspiratory effort of specified magnitude. In different experiments the following can be varied. 1. Inspiratory effort; 2. Resistance added proximal to point of measurement; 3. Resistance added distal to point of measurement; 4. Lung volume. By employing various combinations of these parameters, an attempt will be made to estimate the total power of the muscles of breathing and the proportionaL dissipation of this power on internal external impedance.